Rechargeable cells suffer from self-discharge so must be topped up by periodic charging. Rechargeable cells also have finite charge/discharge lifetimes and can suffer from long term chemical degradation even when not used. This can present reliability issues and so they are not suitable for applications where the power supply must be guaranteed to be available on demand and/or at very short notice.
Primary (non-rechargeable) cells can be configured so that they are inert until required for use then activated to provide power. No chemical reactions are taking place when the cells are in the standby state so shelf life and in-service times can be very long (10s of years).
One example of a standby cell is the thermal battery (as used in ejection seats). In these devices the electrolyte is solid in the standby state and the battery is activated when the electrolyte is melted by a pyrotechnic device.
A further example of a standby cell is a salt water activated battery as used for submarine weapons such as torpedoes. The electrolyte is not present in the standby state and the battery is activated by introduction of a suitable fluid. The electrolyte can be stored externally and activation is achieved by injection into the cell or alternatively the electrolyte, or critical parts of it, can be supplied from an external source.